Answers to: Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used? go into substantial detail explaining this determination. These have been helpful, which encouraged me to try to read the linked paper again. However, based on comments and the paper itself, I see that assigning limits to what sounds to me to be an emission lag of several seconds between the end of the merger and a subsequent gamma ray emission from the same general object/area had to be at least considered and included in some way.
Here I'd like to ask only about this lag. While I'd also like to know what produces the time-lagged gamma ray emission I'm not sure a priori if that is a different question and/or if it's been answered already.
Question: Why can there be several seconds of lag between a binary neutron star merger and the emission of gamma rays from the same region?